1. Field of the Invention
The present invention relates to a demodulator for demodulating a signal modulated by frequency shift keying (FSK) and outputting the demodulated received data.
2. Description of the Related Art
In an FSK-modulated signal, ‘0’ and ‘1’ bits are represented by two different frequencies centered on a carrier frequency represent. A typical FSK receiver selects a desired channel from a range of high-frequency signals, converts the selected channel to an intermediate frequency, and discriminates between the ‘0’ and ‘1’ frequencies at the intermediate frequency level to obtain the received data. Conversion of the received FSK signal to the intermediate frequency (IF) signal can be effected as shown in FIG. 1, by use of a mixer 10 and a local oscillator 11 tuned to a frequency slightly above or below the channel carrier frequency.
Japanese Patent Application Publication No. 11-298359 describes a double-converter FSK receiver in which the received signal is converted using a first local oscillator with good stability over a wide frequency band, then converted again using a high-resolution second local oscillator that can be tuned within the tuning steps of the first-stage local oscillator.
A general problem in these conversion schemes is that there is a risk of reversing the logic of the data. Consider, for example, a carrier signal with a center frequency fc of 314.95 MHz, in which the frequency fc+30 kHz represents ‘1’ and fc−30 kHz represents ‘0’. If this signal is down-converted to an intermediate frequency of 80 kHz by use of a local oscillator tuned to a lower local frequency fi of 314.87 MHz (80 kHz below fc), then after the conversion, ‘1’ will be represented by 110 kHz and ‘0’ by 50 kHz. If the same signal is down-converted to 80 kHz by use of a local oscillator tuned to an upper local frequency fi of 315.03 MHz (80 kHz above fc), then after the conversion, ‘1’ will be represented by 80 kHz and ‘0’ by 110 kHz. The logic levels in the demodulated data are accordingly reversed, depending on whether the local oscillator is tuned to a frequency above or below the carrier frequency.